Semiconductor diode lasers with a ring resonator are an alternative to linear-cavity (Fabry-Perot) lasers for photonic integrated circuits and other applications. A ring resonator utilizes total internal reflection in a circular ring structure thus eliminating the need for end-facet mirrors and allowing direct integration with waveguides, and other photonic integrated circuit components. Previous semiconductor ring diode lasers have operated in a bidirectional mode characterized by lasing in the two counterpropagating ring directions (clockwise and counterclockwise). This bidirectional mode of operation is undesirable for the following reasons: (1) the lasing power in the ring cavity is distributed between the two lasing ring directions so that only half of the total circulating laser power can be accessible to a single output waveguide when a branching Y-junction output coupler is used (the remaining half of the laser power is propagating around the ring cavity in a direction which is not coupled out at the Y-junction); (2) the lasing power in the non-output-coupled ring direction can actually be much larger than that in the output-coupled direction as a result of back-injection by a reflection of the output-coupled lasing at an end-facet mirror at the termination of the output waveguide or by back-reflections from other downstream photonic integrated circuit components (we have shown that such back-injection can reduce the power and slope efficiency of the output-coupled light by as much as an order of magnitude); and (3) bidirectional ring diode lasers exhibit modal instabilities and kinks in their light-vs-current characteristics due to strong coupling of the counterpropagating lasing modes through the semiconductor gain medium.
The attainment of unidirectional operation in a semiconductor ring diode laser would eliminate many of the undesirable characteristics of bidirectional ring diode lasers as discussed above. Since the majority of the lasing power in a unidirectional device would be in a single preferred ring direction, it would be more readily accessible to a single output-coupled waveguide, thereby leading to an increase in the output power. And since any reflection feedback would be coupled back into the ring cavity in the unsupported low-gain direction, it would have much less of an effect on the output-coupled lasing. Unidirectional operation will also improve the operating characteristics of ring laser sources for use in photonic integrated circuits and other applications by reducing noise and instabilities due to gain competition between counterpropagating lasing modes. This results in much more linear and kink-free light-vs-current characteristic in unidirectional ring diode lasers. And finally, the development of a monolithic unidirectional ring diode laser will lead to new applications based on the control and switchability of the direction of lasing in the ring. These applications include but are not limited to ring gyroscopes (in which a beat frequency between two oppositely-directed unidirectional ring diode lasers is detected to measure the rotation rate), optical logic elements (in which the direction of lasing in the ring is the logic state of the device; and this direction is electrically controlled and switched), optical signal routing elements for photonic integrated circuits (in which the signals are routed around a ring cavity with multiple outputs which can be selected by electrically or optically controlling the ring direction and other electrically switchable components such as output-couplers and waveguides and other optoelectronic elements as may be required).
The attainment of unidirectional lasing has long been a goal in the development of ring diode lasers. Unidirectional lasing in a ring laser cavity requires the use of a nonreciprocal gain or loss (attenuation) mechanism which favors one direction of propagation of the lasing light over another. Previously, the mechanisms proposed for achieving unidirectional lasing in ring diode lasers have been based either on reflection from a cleaved end-facet mirror or on injection locking by a source of lasing radiation external to the ring cavity. The previous reflection schemes relied on the angular-dependent reflection from the cleaved- or etched-facet output mirror in a triangular or polygonal ring diode laser, or on the back-reflection from a cleaved-facet mirror located at the termination of one of the output waveguides of a circular or square ring diode laser. The laser radiation for the injection locking scheme was injected into the ring at a partially transmitting etched-facet ring cavity mirror. The source for injection locking was either another laser or else the lasing output from the ring laser itself, in which case an external etched-facet mirror was used to reflect a part of the lasing output back into the ring cavity in a preferred direction of propagation.
The common requirement of all the previous schemes for unidirectional lasing as presented above is the need for etched- or cleaved-facet mirrors. The requirement for these mirrors complicates the fabrication of these devices and limits their reliability due to the exposed facet which must be coated to protect it from the environment. These facet mirrors also prevent the unidirectional ring diode laser from being directly coupled into other waveguides as for the fabrication of a photonic integrated circuit. These previous devices must instead be air-gap coupled which is less desirable since it introduces transmission and coupling losses and is very sensitive to fabrication tolerances (the air-gap spacing must be precisely controlled since this air gap forms an additional resonant cavity).